1. Field of the Invention
The present invention relates particularly to a heat conductive silicone rubber composite sheet that is suitable as a heat dissipating member provided between a heat generating electronic component and a heat dissipating component such as a heat dissipating fin, wherein the heat conductive silicone rubber composite sheet has good electrical insulation and thermal conductivity, and also has excellent strength and flexibility.
2. Description of the Prior Art
Conventionally, electrically insulating heat conductive materials are used as heat dissipating members for heat generating electronic and electrical components such as power transistors, MOS transistors, FETs, thyristors, rectifiers and transformers. Examples include materials produced by adding a metal oxide powder such as beryllium oxide, aluminum oxide, aluminum hydroxide, magnesium oxide or zinc oxide to a synthetic rubber such as silicone rubber (refer to Japanese Laid-open Patent publication (kokai) No. Sho 47-32400 (JP47-32400A)), and materials in which boron nitride is added to a silicone rubber and the rubber is reinforced with a network type insulating material (refer to Japanese Laid-open Utility Model publication (kokai) No. Sho 54-184074).
Furthermore, one method of further improving the thermal conductivity of the aforementioned heat dissipating member is by keeping the thickness of the member as thin as possible. However if the thickness is reduced too much, a problem arises in that the strength, durability, or electrical insulation of the heat dissipating member is lost. As a solution to this problem, multi-layered structures that utilize a film with excellent heat resistance, electrical insulation and mechanical strength, such as an aromatic polyimide, a polyamide, a polyamideimide, or a polyethylene naphthalate, as an intermediate layer, and a silicone rubber layer containing added beryllium oxide, aluminum oxide or aluminum hydroxide or the like, which provides excellent thermal conductivity and electrical characteristics, as an outer layer have been proposed, and for example, Japanese Post-Examination Patent publication (kokoku) No. Hei 2-24383 (JP2-24383B) discloses a heat conductive electrical insulation member with a laminated structure comprising at least three layers, comprising a polyimide(amide) film containing a predetermined quantity of aluminum oxide or the like as an intermediate layer, and silicone rubber layers containing added aluminum oxide or the like as a pair of outer layers disposed on both surfaces of the intermediate layer.
However, in these heat conductive electrical insulation members with multi-layered structures, the adhesion between the silicone rubber layers of the outer layers and the film of aromatic polyimide or the like of the intermediate layer is unstable, and over time, interlayer separation is likely, meaning the members suffer from durability problems.